The major difference between XP Pro 32-bit (x86) and XP Pro 64-bit (x64) is the amount of memory it can address. 32-bit can address a little over 3 GB of RAM, wheras the 64-bit can address up to 6 GB. That's the major difference I've seen (I used to have both on my PC, now I'm just on x64).
That's all I know - everything I use on XP x64 I haven't had too much of a problem with.
EDIT - the people in your identical thread under Windows on the Mac have more experience with programs then I have - stick with them.